1. Field of the Invention
The present invention relates to a method for recovering methane gas from natural gas hydrate, more specifically, recovering methane gas through an injection of gas mixture containing air and carbon dioxide oxygen.
2. Description of the Related Art
Natural gas may be an eco-friendly energy source that plays an important role in the energy system which has been maintained without destruction of the natural environment. Natural gas exists as natural gas hydrate in nature, which is a crystalline clathrate hydrate containing hydrocarbons such as a major component of methane, and trace amount of ethane, propane and butane.
Natural gas hydrate deposited worldwide is being noted as an alternative energy source to fossil fuel in the future, since natural gas in a form of natural gas hydrate has been reported to be stored in an amount of about 0.2×10^15 and 7,600×10^15 m3·S. Such amount is expected to cover most of the energy being used worldwide (See: Sloan, Jr. E. D., et al., Nature, 426:353-359, 1998; Lee, S. Y., Holder, G. D., Fuel Processing Technology, 71:181-186, 2001).
In order to use such valuable natural gas hydrate in daily lives, methods for dissociating CH4 gases from the natural gas hydrate should be developed and several approaches have been made in the art as follows: A thermal stimulation sends high temperature water through a pipe and dissociates methane from natural gas hydrate; Depressurization, the vacuum device decreases the pressure between the natural gas hydrate layers to dissociates methane; And injection of inhibitors which dissociates methane from natural gas hydrate by interrupting the condition where the natural gas hydrate can remain stable to change the equilibrium with increased pressure and lowered temperature. (See: Gunn, D. A., et al., Terra Nova, 14:443-450, 2002).
When the above methods are used, however, methane that is most part of the natural gas hydrate gets released unconditionally. Such situation creates the corruption of the natural gas hydrate layers and causes geological problems such as ground subsidence. As a consequence, the ecosystem will be negatively affected. (See: Lelieveld, J., et al., Nature, 355:339-342, 1992).
Hence, a study to develop the most efficient means to recover methane without dissociating the natural gas hydrate layers is required. The patent the inventors registered Nos 10-0735841 ‘method for recovering methane gas fro natural gas hydrate’ discloses a method to effectively recover methane by reacting the flue gas that contains carbon dioxide and nitrogen with hydrate gas.
However, when the method in the registered patent is being used, one needs to consider that there is a limit to develop deep-sea hydrate gas (a hydrate gas that has the temperature above 10 to 15° C. as a result of geothermal heat) since the safety is low based on the temperature. There has been a problem where it was hard to replace methane with the flue gas at high temperature.